1. Field of the Invention
The present invention relates to an MRI (magnetic resonance imaging) apparatus and a magnetic resonance imaging method which excite nuclear spins of an object magnetically with an RF (radio frequency) signal having the Larmor frequency and reconstruct an image based on NMR (nuclear magnetic resonance) signals generated due to the excitation, and more particularly, to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which make it possible to acquire a frequency spectrum of magnetic resonance signals from a metabolic product in the object by MRS (Magnetic Resonance Spectroscopy).
2. Description of the Related Art
Magnetic Resonance Imaging is an imaging method which excites nuclear spins of an object set in a static magnetic field with an RF magnetic signal having the Larmor frequency and reconstruct an image based on NMR signals generated due to the excitation.
In recent years, 1H-MRS is performed as an efficient means for early diagnosis of disease (especially, determination of the extent of a tumor). 1H-MRS uses a magnetic resonance phenomenon to observe a frequency spectrum of magnetic resonance signals from a minute metabolite containing 1H nuclei in a living body, and can observe NAA (N-acetylaspartate), Cho (choline), PCr/Cr (creatine phosphate/creatine), Glx (glutamic acid and glutamine), Lac (lactic acid), ml (myoinositol) and the like. The concentration of a proton metabolite such as NAA in a living body is small by about quadruple digits compared to that of biological water and metabolites distributed within 5 ppm. Therefore, it is important, in 1H-MRS, to properly set the voxel size to be an MRS target and the number-of-times-of-integration of echo signals for generating a spectrum so as to acquire a spectrum having narrow line widths with an improved S/N (signal to noise) ratio. The same applies to other MRS processes such as 31P-MRS and 13C-MRS.
Note that, as the related art of MRS, a technique has been known that alternatively performs plural pulse sequences for acquiring magnetic resonance data with regard to the concentrations of labeled glucose, unlabeled glucose, labeled compound and unlabeled compound (see, for example, Japanese Patent Application (Laid-Open disclosure) No. 2000-14657). According to this technique, the metabolic rates of glucose and its compound can be measured without blood sampling (which would be very burdensome for the object).
However, the voxel size and the number-of-times-of-integration had been conventionally set depending on experience of an operator though it is important to properly set the voxel size and the number-of-times-of-integration in MRS as described above. Accordingly, it is difficult to properly set the voxel size and the number-of-times-of-integration when the sensitivity of the RF coil or the susceptibility of the object is not uniform. As a result, an actual examination may result in failure due to a body motion or an examination time may increase unnecessarily. Accordingly, there is an issue of unnecessarily increasing an examination time.
These problems can not be solved even by the above-described conventional technology.